Electrohydraulic servo mechanism



Jan. 13, 1953 w. c. MOOG, JR 2,625,135

ELECTROHYDRAULIC SERVO MECHANISM Filed April 26, 1950 2 SHEETS-SHEET 1 INVENTOR WILLIAM c. M006 JR.

BY W

ATTORNEY Jan. 13, 1953 w. c. MOOG, JR 2,625,135

ELECTROHYDRAULIC SERVO MECHANISM Filed April 26, 1950 2' SHEETS-SHEET 2 INVENTOR' WILLIAM C. MOOG JR.

BY //M$W ATTORNEY Patented Jan. 13, 1953 ELECTROHYDRAULIC SERVO MECHANISM William C. Moog, J r., East Aurora, N. Y., assignor,

by mesne assignments, to Research Corporation, New York, N. Y., a corporation of New York Application April26, 1950, Serial No. 158,268

6 Claims.

This invention relates to a servo-mechanism of the electro-hydraulic type.

A principal object of the invention is to provide a servo control valve wherein an electrical signal is converted to a proportional hydraulic response with a minimum of distortion, a maximum of sensitivity and linearity of response, and an extremely short reaction time, for the control of aircraft and guided missiles, for positioning of heavy artillery and radar antennae, for controlling operations in nuclear and X-ray processes, where human presence would be dangerous, or for other similar purposes.

Another object of the invention is to provide such a valve having a large hydraulic amplification power thereby permitting the use of a relatively small electrical control signal and small electrical actuators.

Another object is to provide an electro-hydraulic servo-mechanism the operation of which is independent of fluctuations or changes in the supply pressure of the pressure fluid within very broad limits.

A further object is to provide an electro-hydraulic valve that is simple as to its parts, light in weight, and readily adjustable to respond to various reaction times, control impulses, and output requirements.

These and other objects and advantages are provided by an electro-hydraulic valve comprising a balanced valve port controlling spool and an electro-hydraulic actuator to vary the force acting on the balanced spool whereby the valve ports are proportionally controlled in response to the magnitude of an electrical control signal.

The invention will be more particularly described with reference to the illustrative embodiments of the invention shown in the accompanying drawings, in which:

Fig. 1 is a longitudinal section illustrating a servo-mechanism constructed in accordance with the principles of the invention;

Fig. 2 is a sectional view-on line 22 of Fig. 1; and

Fig. 3 is a longitudinal sectional view of a modified form of the invention.

Referring to the drawings, 8 is the valve body having pressure fluid ports 9 and i9 and drain or sump port I I. Ports 9 and I connect the valve with the mechanism to be operated, which is shown by way ofillustration as a double acting piston [2, comprising a cylinder l3, and a piston l4, having operating faces I5 and I6 of unequal area. Face l5 has twice the area of face l6.

. Conduit l1 connects valve port 9 to the cylinder between piston face l5 and the adjacent end of the cylinder, while conduit 18 connects valve port 10 to the pressure supply line l9 and to the other end of cylinder [3.

A valve spool 29 is slidably carried within the valve body 8 and comprises a cylindrical body having two control channels or annular grooves 2| and 22. A cantilever spring 23 provided with an adjustment screw 24 biases the spool toward the right. Preferably the cantilever spring has a linear force-deflection relationship.

An annular groove 25, within the valve body. connects spool channels 21 and 22, when the spool is centered in the valve body as shown in the drawings, providing orifices 26 and 21 which control the flow of pressure fluid within passages 28, 29 and 30. Thus with the spool centered in the valve body orifices 26 and 21 provide equal resistance to the flow of pressure fluid between supply passage 28, and passage 29, and between passage 29 and drain passage 30. Under these conditions the pressure in passage 29 will be the mean of the supply pressure in passage 28 and the drain pressure in passage 39. Piston [4 will be at rest, as the pressure fluid in passage 29 is vented to the large side l5 of the piston I4 and the supply pressure is vented to the small side It, of the piston and the hydraulic forces acting on the piston are balanced.

Lateral movement of the spool 20 will unbalance the flow resistance at orifices 26 and 21, formed by annular grooves 2i, 22 and 25, and the pressure in passage-29 will no longer be the mean of the supply and drain pressures. Thus movement of spool 20 toward the left will openorifice 21 and pressure fluid from supply conduit 19 will be vented directly to the large side I5 of piston [4 through passage 28, orifice 21, passage 29, and conduit l1. Concurrently with the opening of orifice 21, orifice 26, vented to the drain passage 30, will be closed, and piston M will move toward the right with a velocity proportional to the area of opening of orifice 21.

Conversely, if the spool is moved toward the right from its centered position, orifice 21 will close shutting off the supply of pressure fluid to the large side I5 of piston I4 and orifice 26 will be opened allowing pressure fluid on the large side of the piston, in conduit l1 and in passage 29 to be vented to the drain port I l, and piston M will move toward the left with a velocity proportional to the area of opening of orifice 26.

When external axial forces are applied to the piston [2, the reactions described above will be slightly modified. For instance, with the piston at rest, the pressure forces must be unbalanced to produce reaction force. This is accomplished by an extremely small departure of the spool 20 from its centered position in orifice 25. Then orifices 26 and 21 will differ in flow restriction to the extent that the pressure in chamber 29 can change to the. value necessary to provide the pressure-force unbalance. Also, external load will afiect the velocity of piston motion proportional to the position of spool 20. This results from the change in pressure difference at either orifice 2B or 21 depending upon the axial load and causes a diflerent flow rate or piston velocity for a particular opening of either orifice.

Lateral displacement of the valve spool 20 is brought about by changes in the fluid pressure acting on the head 3| of thespool against. the force of the cantilever spool biasing spring 23. The pressure of the pressure fluid actin on spool head 3| is controlled by an electric signal through --the eleetro-h-ydraulic actuator 32, comprising a *fluid pressure nozzle '33, a pressure reaction plate -'34 and an electric motor .35.

Nozzle 33 has a restricted opening31 at one end and at the other forms an annular orifice between its tipand pressure reactionv plate 35. A-chamber 38 lies-between orifice 36 and the restricted opening 31. Pressure. fluid is supplied to chamberLSS. of the nozzle through 1passage'39 provided with a flow restrictor 40. Pressure fluid thus suppliedto chamberx38. passes through the restricted openingttl. and, provided: that spool 20' has no. instantaneous velocity; creates in chamber 4 I an equal pressure which acts against the head 3l'0f1SI1001' 2n and a spool deflection proportional .to thepressure within nozzle chamber 38 is obtained. Coinstantaneously the pressure fluid in chamberz38 issues from orifice 36 into chamber .62 that is. nominally substantially filled with liquidandis provided with an upper .drain passage 43.

*The. rate of flow of pressure fluid from chamber 38 and. sump chamber: 42 is determined by the pressure difierencebetween the chambers. pressure :diiierence. is exerted on the area 1 of the reaction plate 34- equal to the tip area of nozzle 33. As. theplate area ;for the iull range of motion of the plate remainsessentially constant, then the pressure in chamber 38 isalways proportional ,to the iorceon the reaction plate .34. .It is the function; of motor 35-to produce on the plate a .force ,proportionaltoan electrical signal applied to the-motor.

- The electric ,motor- 35; of the electro-hydraulic actuator comprises:-= a; coil 4.4;;having terminals "45;.D0le pieces fi and 4T; .permanent;magnets 48 and A9; armature 0, the lower end of which forms the reaction-plate 34; and armaturebiasing spring; 5 I.

-Thepermanent magnets '48 and 49 of the mo- 1 tor are'oriented so that they contribute toward the maintenance of dissimilar poles of pole pieces 46 and 47. An armature mount 52 provides a base for cantilever mounting of thearmature 50. Thearmatureextends through the air gap 53 between the pole pieces 46 andAl. Armature .50 is constructed of low hysteresis, magnetic re- .silientmaterial and has a relatively low-spring rate at the air gap 53 by virtue of its cantilever mountingEZ, and is free .to move am the air gap,

throughan arosubtendedby the pole'piece-fl and orifice 3B.

A biasing spring 51 is secured at one end to "the-reaction plate 3 3 and at the other end to a biasing spring mount 54, having a spring force magnetic field-on the armature.

-.mature rate is substantially linear.

and extends. approximately half way from the center towards both poles; in this range the ar- It will be observed that without the biasing spring the armature would not remain in its centered position the pole pieces 46 or iii.

in the air gap 53, but would snap toward one of This is due to the decrease in magnetic reluctance between the ar- ;mature and the; pole piece as the armature -pole pieceincreases.

from the center toward either pole piece. resultant forces acting on the armature Within .ing the flux .den-sitywithin the air gap53.

against the cantilever spring 23.

moves toward that pole-piece while the ,magnetic reluctance between the-armatureand the other Thus the flux density in the airgap 53 increases as the armature moves The the-operating range are counterbalanced bythe biasing spring, since its spring rate is equal and opposite to the armature rate in the operating .regio LI-the springrate and the armature rate cancel, leaving no rate within the operating range of the air gap and the armature can .bepositionedatany point within the operating range without theapplication-of any substantial force. Wherever thearmature ispositioned in .theopcrating range, it willtend to remain because the magnetic and spring forcessare always in balance. This effect, while not essential to the operation of the valve, is. ofimportancegas, will be .shown hereinafter, as it. permits the operation .of theelectrohydraulic actuator independently of fluctuations or changes in the supply pressure of ,thepressure fluid within very broad limits.

. The coilM actnates the armature 5G. bychang- In theinitia-l operation of the ,servo, valve, the valve spool is centered in the valve body beforeany current is applied to the coil-M. The spool is centered by means of the armature biasing spring tension adjustment screw 55, which changes the biasing spring force. If this force is increased, the armature is forced toward thenozzle orifice, restricting theufiowof pressure fluid from the ori'flce and creating apressurein chambers 33 and 4'] causing a linear displacement of the spool 20 Asthe pressure rises, the pressure force on the reaction plate 34 increases until a balance with the new biasing spring vforce is reached.

The coil 44 may be of the push-pull type wherein the magnetic iorceupon the armature .will. have magnitude and sense proportional to I the difierential current between the two wind- .fall substantially proportionally to the coil current, producing a relative movement in valve spool 20. This becomes apparent upon consideration of all ofthe forces acting'on the "armature and its extension in the direction of the spool axis. These forces are: the force due to the current in the coil 44; the force of the biasing spring 5!; the force created by the pressure in chamber 38; and that due to the position of the armature in the air gap. As hereinbefore described, the spring rate of the biasing spring is equal and opposite to the armature rate within the operating range and they cancel; the forces left that affect the armature are the forces created by the pressure in chamber 38, the current in the coil and the biasing spring. With no current in the motor coil the force of the pressure in chamber 38 tending to move the armature toward the right and the force of the biasing spring acting in the opposite direction will reach equilibrium. The armature will remain stationary in an equilibrium position :and the pressure within the chambers 38 and 4| will be constant. When an electric current is applied to the motor coil, the force of the armature will change and the pressure in chambers 38 :and 4! will rise or fall proportionally to the coil current. The pressure in chamber 38 will not be affected however by changes in the supply pressure of the pressure fiuid within wide limits. Any change in supply pressure will change the flow through orifice 40 and annular orifice 36. Hence the distance between nozzle tip 33 and the reaction plate 34 must change in order that the pressure in chamber 38 remain constant as the flow changes.

" example, the valve may be of the multiple port and the bore in the valve body, means biasing said valve member in the direction of said restricted orifice, means for introducing pressure fluid directly into said chamber, and means for varying the pressure in said chamber, comprising But this distance change does not affect the pressure-force balance on the reaction plate and since the motor force is independent of the position of the plate due to cancellation of the armature and spring rates, no change of pressure in chamber 38 will occur as supply pressure changes.

The fact that the reaction plate 34 has changed position to compensate for variations in the fluid supply pressure does not affect the overall response of the servo-mechanism; a given input signal will actuate the armature 50, and the reaction plate which is the extension of the armature, in the same Way regardless of its position in the operating range because of the balance between the armature rate and the biasing spring rate.

In order to prevent oscillation of the spool due to flow reaction forces on the spool 20 at orifices 26 and 21, restriction 31 is provided between chambers 38 and 4| to damp the flow of pressure fluid to and from chamber 38.

Fig. 3 of the drawings illustrates a modified form of the invention wherein an electric motor 57 having a rotatable armature 58 is utilized to actuate the reaction plate 34. Like parts throughout the illustrative embodiments have similar character designations. In operation of the device shown in Fig. 3, electrical signals are supplied to the motor terminals 59 and the magnetic torque produces a force tending to rotate the armature 5'8, and the pressure reaction plate 3 4 secured to the armature shaft 60; the displacement of the plate 34 along the axis of the valve spool 20 being proportional to the applied current.

From the foregoing description it will be seen that the present invention provides a servo control mechanism wherein an electrical signal is converted to a proportiona1 hydraulic response with a minimum of distortion, and a maximum of sensitivity and linearity of response.

It will be evident that various modifications may be made in the form of the valve without departing from the scope of the invention. For

an orifice communicating with said chamber and in fixed position relative thereto, a pressure reaction member positioned adjacent said orifice, spring means biasing said pressure reaction member in the direction of said orifice, and an electric motor for varying the force of said pressure reaction member in response to an electric signal applied to said motor.

2. An electro-hydraulic servo valve comprising a valve body having pressure fiuid control ports, a valve member movably mounted in said body for controlling the flow of fluid through said ports, a pressure fluid chamber in actuating relationship to said valve member, means biasing said valve member in the direction of said chamber, means for introducing pressure fluid into said chamber and means for varying the pressure in said chamber comprising an orifice communicating with said chamber, and means for restricting the flow of pressure fluid from said oriflce'in response to the magnitude of an electrical control signal, said means comprising a pressure reaction plate adjacent said orifice, an electro-magnetic motor having a permanent magnetic field with a superimposed variable magnetic field produced electromagnetically thereon for varying the force of said plate, a spring producing a force balance with the attraction of that component of the total magnetic field which is due to the permanent magnets magneto-motive force, the rate of said spring being such as to effect said force balance, whereby the net force of the pressure reaction plate is varied in response to an electric signal applied to said motor.

3. A servo valve, electro-hydraulic means for actuating the valve, said means comprising a fluid pressure chamber in actuating relationship to the valve, means biasing the valve in the direction of the fluid pressure chamber, and means for varying the pressure in said chamber comprising an orifice communicating with said chamber, and means for restricting the flow of pressure fluid from said orifice in response to the magnitude of an electrical control signal, said means comprising a pressure reaction plate adjacent said orifice, an electro-magnetic motor having a permanent magnetic field with a superimposed variable magnetic field produced electromagnetically thereon for varying the force of said plate, a spring producing a force balance with the attraction of that component of the total magnetic field which is due to the permanent magnets magnetomotive force, the rate of said spring being such as to effect said force balance, whereby the net force of the pressure reaction plate is varied in response to an electric signal applied to said motor.

4. In an electro-hydraulic servo valve comprisingoax valve body having; pressure fluid control =ports,rand saqvalve member movably 'mounted in a bore-in said body for controlling the flow of fluid through said. ports: a pressure fluid chamber, a

restricted orifice connecting the pressure fluid chamber and the bore in the valve body, means biasing said valve member in the. direction of said restricted orifice, means for introducing pressure fluid. directlyinto said chamber, and-means for varying the pressure in said chamber comprising an. orifice communicating with said chamber and :irr fixed position relative thereto and means for restricting the flow-of pressure fluid from said orifice, said means comprising a pressure reaction "plate adjacent said orificean oscillatory arma- ,tture attached to said plate, spring means biasing said armature in the direction of said orifice, and ganelectro-magnetic motor for varying the force of said armature" iii-response to the magnitude of :an electrical signal applied to said motor.

:5. In an electric-hydraulic servo valve comprising a valve-body having pressure fluid control .ports, and a valve member movably mounted in a a bore in said body-for controllingthe flow of fluid through said ports: apressure fluid chamber, a restricted Orifice connecting the pressure fluid 1 chamber and the bore in the valve body, means biasing said valve member in the direction of said restricted orifice, means for introducing-pressure fluiddirectly into said-chamber, and means for varying the pressure in said chamber comprising an orifice communicating with said chamber and in fixed position relative thereto and means for restricting the flow of pressure fluid from said orifice, said means comprising a pressure reaction plate adjacent said orifice, a rotatable armature attached to said-plate, spring means'biasing said armature in the direction of said orifice, and an electro-magnetic motor for varying the force of said armature and pressure reaction plate in response to the magnitude of an electrical signal applied to said motor.

6. An electric hydraulic servomechanism comprising a body portion, a pressure responsive member carried by said body portion, a pressure fluid chamber in actuating, relationship to the Cir pressure responsive member, means biasing said pressure responsive member in the direction of said chambenand means for varyingthe pressure in. said chamber comprising an orifice communicating with said chamber, and means for restricting the flow of pressure fluid from said orifice in response to the-magnitude of an electrical control signal, said means comprising a pressure reaction plate adjacent said orifice, an electromagnetic motor having a permanent magnetic field with a superimposed variable magnetic field produced electromagnetically thereon for varying the force; of said plate, a spring producing a force balance with the attraction of that component of the total magnetic field which is due to the permanent magnets magneto-motive force, the rate of said spring being such as to effect said force balance, whereby the net force of the pressure reaction plate is varied in response to an electric signal applied to said motor.

WILLIAM c. MooG, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,754,410 Allwill Apr. "15, 1930 2,261,827 Brown et al. Nov. 4, 1941 2,520,115 Cahill et a1 Aug. 29, 1950 FOREIGN PATENTS Number Country Date 174,396 Great Britain Jan. 16, 1922 98,885 Switzerland May 1, 1923 112,466 Switzerland Nov. 2, 1925 217,121 Switzerland .4 Feb. 16, 1942 

